This invention relates generally to an improved particle accelerator and in particular to an improved high intensity electron gun or accelerator having a large area beam cross-section. More specifically, the invention relates to an improved method and apparatus for tensioning the foil covering for the exit window apertures for such an accelerator.
In U.S. Pat. No. 3,863,163, which patent is incorporated herein by reference, there is described in electron gun of the type having cathode and grid assemblies, a vacuum-tight housing for the cathode and grid assemblies, and a multi-apertured anode support plate which forms one wall of the vacuum-tight housing. An electron transparent foil covers the exterior surface of the anode support plate and is sealed about its circumference to the support plate to perfect the vacuum.
One problem which has arisen in the manufacture of such electron guns is that the foil is typically made of either rolled titanium or aluminum and during the rolling process, the thickness of the foil is not made entirely uniform so that tensions and stresses are produced in the foil causing it to be nonplanar. When the foil is placed over the exit window apertures during fabrication of the electron gun assembly and is sealed to the support plate, there exist no means for drawing it uniformly taut over the apertures because of this nonplanar condition of the foil. When the vacuum is drawn within the housing, the foil is, of course, pulled against the exit window apertures and the support plate. However, because the foil was nonplanar prior to the application of the vacuum, tiny folds or creases are produced in the foil which, due to metal fatigue and other like mechanisms, cause leaks to be produced in the foil, thereby destroying or greatly reducing the vacuum tightness of the housing.